The invention relates to a semiconductor device comprising a semiconductor chip encapsulated in synthetic resin, which chip is provided with a passivating film and a polyimide film which is provided between said passivating film and the synthetic resin.
At present, the vast majority of semiconductor chips or ICs are constructed with a synthetic resin envelope, which synthetic resin is termed encapsulating synthetic resin. In said construction, the chip is completely embedded in synthetic resin. Due to the fact that the synthetic resin and the silicon have different thermal expansion coefficients (TEC), variations in temperature may induce large mechanical stresses in the semiconductor device, in particular when the dimensions of the chip are relatively large as in the case of LSIs (large-scale integrated circuits) and VLSIs (very large-scale integrated circuits). The encapsulating synthetic resin generally consists of an epoxy resin filled with quartz powder or glass powder. The silicon chip is provided with a passivating film on the side of the electronic circuit to preclude the circuit from being damaged and contaminated during further treatment and during its life cycle. Such a passivating film consists, for example, of one or more films of phosphor silicate glass (PSG) or silicon nitride. Due to differences in TEC and the consequential stresses microcracks may be formed in said passivating film. Said stresses and microcracks may also lead to damage to the electronic circuit of the chip, thereby causing functional defects and even failure of the chip. Ambient humidity may reach the chip via the microcracks and cause corrosion. Said defects are customarily reduced by applying a so-called stress-relief film to the passivating film. A synthetic resin such as polyimide and silicone resin is used as the stress-relief film.
A semiconductor device of the type described above is known from European patent application EP-A-0 349 001. An unspecified polyimide film is mentioned as the stress-relief film. A disadvantage of the customary polyimides is the presence of large numbers of cracks in the passivating film after the semiconductor device has been subjected to a thermocycling test. A thermocycling test gives an indication of the reliability of the semiconductor device under operating conditions. The presence of cracks may lead to functional failure of the chip. In a customary thermocycling test 200 semiconductor devices are subjected to a number, for example 1000, of cycles of maintaining the temperature at -65.degree. C. for 30 minutes followed by rapidly heating to 150.degree. C., also for 30 minutes, and conversely. After said test, the percentage of semiconductor devices which have functionally failed is determined. Additionally, in a parallel test the number of cracks in the passivating film is determined per chip after a number of cycles.
In the case of temperature variations, five materials and four interfaces are important, namely the interface between the silicon chip and the metal lead frame; the interface between the synthetic resin envelope and the metal lead frame; the interface between the passivating film and the polyimide film and the interface between the polyimide film and the encapsulating synthetic resin. The stresses occurring in the films and along the interfaces depend on the relevant TEC-values and the moduli of elasticity (E). Additionally, at the location of the metal tracks of the electronic circuit a sixth material plays a part.